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RE: SDR Prototype transponders

Hi Trevor

> I noticed that the transponder bandwidth was given as just 45 kHz, what 
> the reason for restricting it to this bandwidth rather than the 100 or 
> kHz that has been used on previous satellites ?

As Bob has said this is a limitation of the PC soundcard and/or driver. The 
Cirrus Logic CS4272 CODEC I have used in the STELLA hardware supports a 
~180kHz passband. PC soundcards may have either hardware limiting analogue 
filtering or are driver limited (by fiddling with the CODEC's digital 
filter)  down to 22kHz (supporting a 44kHz passband) or so despite some 
having sampling rates of up to 192kHz.

As well as the DSP algorithms themselves, among the other things we 
considered at the outset when designing the STELLA DSP IF was the power 
budget, and how easy it would be to drop into existing designs.

We limited the power requirement to 1W and the input and output was set at 

The power limitation has already made a complete and fundamental hardware 
redesign necessary, which is why we coincidentally ended up using the low 
power requirements of the QSD/QSE design like the SDX. Originally we used 
ADC's and DAC's running at the IF and digital up/downconverters in the 
STELLA design. These proved to have too high power consumption, and are an 
additional complication to program.

The fixed 10.7MHz IF interfaces perfectly with both David G0MRF's 
transponder design and as a LEILA replacement in P3 designs. STELLA itself 
is not frequency agile: to provide this would require additional hardware 
that we didn't see as necessary given our other design criteria.

73, Howard G6LVB
Sent via amsat-bb@amsat.org. Opinions expressed are those of the author.
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